Musculoskeletal injuries account for about 3 million occupational injuries annually in the United States. Shane Shapiro, Worker's Compensation Claims Related to Musculoskeletal Injuries, 57 NORTHEAST FLORIDA MEDICINE 41, 43 (2006). Musculoskeletal injuries are common in repetitive physically intensive work. Repetitive motion injuries (e.g. sprains, strains, tendinitis, tendosynovitis, and carpal tunnel syndrome) are a major concern today. Sprains, strains, tendinitis and carpal tunnel syndrome accounted for 41 percent of the injuries and illnesses resulting in days away from work in 2010. Bureau of Labor Statistics, U.S. Dep't of Labor (2011). Over 30,000 work-related repetitive motion injuries are reported annually in private industry in the United States. Bureau of Labor Statistics, U.S. Dep't of Labor (2008). Repetitive motion injuries have the highest median recovery time of all workplace reported injuries and illnesses, with a median twenty-four work days lost for recovery. Bureau of Labor Statistics, U.S. Dep't of Labor (2011). One of the top ten causes of disabling workplace injuries and illness, repetitive motion injuries account for over $1.8 billion in annual workers compensation costs. Liberty Mutual 2010 Annual Workplace Safety Index. Upper extremity injuries are prevalent in repetitive hand intensive work. See Z. J. Fan et al., Quantitative Exposure-response Relations between Physical Workload and Prevalence of Lateral Epicondylitis in a Working Population, 52 AM. J. INDUS. MED. 479-90 (2009); B. Silverstein et al., The Natural Course of Carpal Tunnel Syndrome in a Working Population, 36 SCAND. J. WORK ENVTL. HEALTH. 384-93 (2010).
Yet there is currently no practical way to objectively and efficiently measure repetitive motion exposure. See Z. J. Fan et al., supra; A. Garg & J. Kapellusch, Consortium Pooled Data Job Physical Exposure Assessment, 17TH WORLD CONGRESS IN ERGONOMICS, Beijing, China, (2009); K. T. Hegmann et al., Prospective Cohort Study of Upper Extremity MSDs Among 17 Diverse Employers, 17TH WORLD CONGRESS IN ERGONOMICS, Beijing, China (2009); and B. Silverstein et al., supra. For example, for measurements of upper limb kinematics, current methods involve either direct measurements using instruments attached to a worker's hands or arms, or indirect observations. A commonly employed observational method for quantifying the degree of repetition in upper extremities is the Hand Activity Level (HAL) scale, which is based on the ten point visual-analog scale. W. A. Latko et al., Development and evaluation of an observational method for assessing repetition in hand tasks, 58 AM. INDUS. HYGIENE ASS'N J. 278-85 (1997). HAL is a factor used to ascertain the threshold limit value for repetitive manual tasks. AM. CONF. OF GOVERNMENTAL INDUS. HYGIENISTS, HAND ACTIVITY LEVEL TLVS® AND BEIS® BASED ON THE DOCUMENTATION OF THE THRESHOLD LIMIT VALUES FOR CHEMICAL SUBSTANCES AND PHYSICAL AGENTS & BIOLOGICAL EXPOSURE INDICES, 196-98 (2009). The HAL scale ranges from 0 to 10, and quantifies repetitive motion ranging from idle hand activity (0) to rapid steady motion with difficulty keeping up (10). Under current methods, a trained observer calculates HAL using either subjective estimation or objective measurements of frequency and duty cycle from a stopwatch time study or a manual frame-by-frame video analysis.
HAL is based on duty cycle and exertion frequency. For many tasks, these are associated with hand and arm movements and static or dynamic exertions. The frequency is the rate of repetition in cycles per second or Hz. The duty cycle is the percent ratio of hand exertion time to the total cycle time of the activity (i.e. exertion time/cycle time). Consequently a 100 percent duty cycle refers to repetitive work where force is exerted for the entire time and a 50 percent duty cycle occurs when force is exerted half of the time.
Current methods of measuring and calculating limb kinematics have been mostly limited to research studies and are highly impractical for routine occupational health and safety practice. The conventional approaches for rating repetitive limb activity based on observable characteristics of manual work are inaccurate, cumbersome or impractical for routine work evaluation and have been limited primarily to research laboratories. Compared to instruments, observation is non-invasive but lacks precision and accuracy, is not suitable for long observation periods, and requires considerable analyst time. B. D. Lowe, Accuracy and Validity of Observational Estimates of Shoulder and Elbow Posture, 35 APPLIED ERGONOMICS 159-71 (2004). Alternatively, attaching sensors on working limbs is time consuming, and sensors may interfere with normal working operations. Thomas Y. Yen & Robert G. Radwin, Comparison Between Using Spectral Analysis of Electrogoniometer Data and Observational Analysis to Quantify Repetitive Motion and Ergonomic Changes in Cyclical Industrial Work. 43 ERGONOMICS 106-32 (2000). Not only is existing instrumentation use resource intensive, but the required technical knowledge often makes this approach inaccessible to general industry.